1. Field
This disclosure relates to a method for removing sulfur from liquid fuels while operating at mild conditions (close to ambient) and by utilizing catalysts and adsorbents. The method is particularly suited for treating fuels for use in fuel processors associated with fuel cell power systems.
2. General Background
Sulfur removal from liquid hydrocarbons such as gasoline and diesel is an area of great interest due to the Environmental Protection Agency's mandate that the sulfur in gasoline should not exceed 30 ppm. In the case of diesel, regulations call for a reduction from 500 ppm to 15 ppm. This translates to an almost tenfold reduction in the current sulfur content from present levels. Sulfur reduces the life of noble-metal-based catalytic converters as it tends to form stable compounds with the active catalyst components. Sulfur also oxidizes to sulfur oxides, which are detrimental to the environment.
For fuel cell applications, sulfur is a poison to reforming catalysts, water-gas shift catalysts and noble metal catalysts that are used in the process train of a fuel processor. Sulfur also poisons the anode catalyst in the PEM fuel cell. The sulfur concentration in the fuel that enters the hydrogen generation system should therefore be less than 1 ppm for PEM applications and less than 30 ppm for Solid Oxide Fuel Cell (SOFC) applications. Per military standards (MIL-T-5634M/N), the maximum amount of total sulfur content in logistic fuels is 0.3 wt. % and therefore requires treatment prior to fuel processing.
Desulfurization of military logistic fuels such as JP-8 and Diesel (NATO-F76 Navy Distillate) is of vital importance for the deployment of shipboard (or on-board) hydrogen generators for fuel cell power systems. Well-known desulfurization methods such as hydro-desulfurization are not suitable for shipboard (or on-board) applications, since a means for hydrogen supply such as electrolysis is required. The “deep” sulfur compounds such as the benzothiophenes can be converted to lighter sulfur compounds such as H2S by operating the fuel processor at high temperatures (800° to 900° C.—ATR units); the lighter sulfur compounds are then removed by using ZnO based adsorbent beds.
On a commercial scale, sulfur in fuels is removed by the hydro desulfurization (HDS) process. HDS requires pure hydrogen to be co-fed along with the fuel to prevent catalyst deactivation. The gas (hydrogen)-liquid (fuel) reaction is conducted over a solid catalyst at 300° C. to 350° C. and 50 to 100 bar, and is limited by mass transfer resistances. Vapor phase HDS has been conducted over catalysts such as supported molybdenum carbides and nitrides in the laboratory at 420° C. and ambient pressure, but the long-term stability of these catalysts remains to be determined. (M. E. Bussell, K. R. McRea, J. W. Logan, T. L. Tarbuck, J. L. Heiser, J. Catal., 171, p 255, 1997.)